The present invention relates generally to aconical scan microwave antenna for transmitting a pencil beam or receiving a plane wavefront. More particularly, this invention relates to a microwave antenna having a stationary and fixed primary reflector consisting of an axially symmetrical surface of revolution, and a spherical wave point source feed-and-subreflector assembly on the same side as the focus of the primary reflector. The subreflector and feed are positioned outside of the aperture of the plane wavefront reflected from the primary reflector and the surface of the subreflector is shaped such that the reflection off of the subreflector of the spherical wave from the feed strikes the primary reflector and is reflected thereby in a collimated co-planar wavefront from each point on the primary reflector, which is potentially scannable by rotating the subreflector-and-feed assembly about the axis of the primary reflector. Further, neither the feed nor the subreflector aperture blocks any portion of the reflected pencil beam at any position of the subreflector-feed-assembly about the axis of the primary reflector.
In the past it has been common to scan using a limited portion of a primary reflector having a surface consisting of a surface of revolution. For example, the patent to Graham, U.S. Pat. No. 3,792,480 discloses the use of a portion of a paraboloid and a portion of a hyperboloid for the primary and subreflectors to produce a collimated beam. Such portions of a paraboloid or a hyperboloid, or in fact any axially symmetrical surface of revolution, are extremely difficult to fabricate to the tolerances needed for very long range directionally accurate microwave transmission and reception, e.g., from a satellite in space orbit to earth. In addition, the subreflector is located on the axis of the surface of revolution, of which a portion is being used as the primary reflector. This limits the amount of the portion of the surface of revolution which can be used at a given time as the primary reflector without aperture blockage by the subreflector of the beam being transmitted or received. Not all of the surface of revolution can be used at once, thus decreasing, e.g., the antenna's gain and directivity. The patent to Gans, U.S. Pat. No. 4,145,695, discloses an offset subreflector Cassegrainian antenna using a paraboloid primary reflector with an astigmatic launcher in the path between the feed and subreflector, with the astigmatic launcher having a surface shaped to compensate for astigmation due to off axis scanning of the beam from the primary reflector.
Rotation of the subreflector to achieve conical scanning of the beam from a parabolic primary reflector is also known in the art as shown in the patent to Feldman, U.S. Pat. No. 2,419,556. However, severe aperture blockage exists in the design shown in the patent to Feldman because the subreflector is located in the beam aperture which is coaxial with the axis of the primary reflector.
The problems enumerated in the foregoing are not intended to be exhaustive but, rather, are among many which tend to impair the effectiveness of previously known off-axis scanning microwave antennas with the primary reflector formed by a surface of revolution. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that scanning microwave antennas existing in the art have not been altogether satisfactory.